A ceramic substrate having aluminum nitride as a main component has excellent insulation characteristics and heat dissipation properties. A metal conductor layer is bonded to this ceramic substrate having aluminum nitride as a main component by an active metal method, a direct bonding method, or the like to form a circuit board. This circuit board is used as a circuit board for a high power semiconductor, such as a power transistor. For power transistors, a plurality of power transistor chips (hereinafter also referred to as chips) are incorporated in one package to form a power transistor module (hereinafter also referred to as a module).
The power transistor is used at high power, and therefore, an amount of heat generation of the chip is large. In recent years, due to miniaturization of the module, a size of the chip has also become small, and an amount of heat generation per unit area has tended to become larger.
When the chips generate heat in this manner, the entire module thermally expands. At this time, since an end portion of the module is fixed to a heat dissipating fin or the like, bending moment occurs in the entire module in use of the power transistors. Therefore, there is a problem that if the ceramic substrate used for insulation of the chips from each other has weak strength, a crack occurs in the substrate and insulation failure occurs in the module.
In addition, in these days, a lead-free solder is often used for soldering the chip and the like to the ceramic substrate for consideration of environment. Generally, the lead-free solder has a higher melting point than a lead-containing solder. Therefore, when the lead-free solder is used, soldering temperature increases regardless of whether in a reflow method or a flow method.
Further, it is also necessary for a solder used for bonding between a base metal of the module and the ceramic substrate to be lead-free. This base metal-ceramic substrate soldering provides a largest soldering area in the module. Therefore, when reflow temperature increases, bending moment applied to the module also becomes very large due to a difference in coefficient of linear expansion between the base metal and the ceramic substrate. Therefore, when the ceramic substrate has weak strength, there is a possibility that a crack occurs in the substrate also during assembly of the module.
In this manner, in addition to insulation properties and heat dissipation properties, high strength is also required of a ceramic substrate used for a power module.
Regarding this, Japanese Patent Laid-Open No. 2007-63122 (Patent Document 1) discloses an aluminum nitride substrate having a high thermal conductivity and excellent heat dissipation properties. In addition, Japanese Patent Laid-Open No. 2003-201179 (Patent Document 2) discloses an aluminum nitride substrate having high heat dissipation characteristics and mechanical strength.